A. Field of the Invention
This invention relates to solid gas generants particularly those suitable for the production of substantially pure nitrogen gas, more particularly to the use of alkali metal azides as a base for such systems, processes for their preparation and use as well as to systems employing such gas generants in their operation.
B. Description of Prior Art
With the realization that in private automotive vehicles crash restraint systems requiring positive effort on the part of the user for effectiveness were ignored by a substantial portion of the population intended to be protected, professional and governmental safety programs have tended to emphasize automatic restraint systems, such as crash bags.
The details of crash bag systems have been widely discussed, as have the reasons for selection of pyrotechnic devices when rapid dependable gas supplies therefore are required. The operational constraints of crash bags are also well known. The system must supply absolutely non-toxic gas to inflate the bag because some bag systems vent into the passenger compartment on deflation and because of the very real probability of bag rupture in an actual crash situation. Naturally the gas must inflate the bag at a temperature which the beneficiaries of the protection can tolerate. The time period for attainment of maximum inflation has been determined to be from 20 to 100 milliseconds, preferably 20 to 60 milliseconds. The device must be safe to handle and store prior to production. It must be adaptable to mass production line installation techniques and not introduce an unreasonable hazard then or during the life of the vehicle. It must assure reliable operation during the life of the vehicle containing it, which may be 10 years or longer.
The objectives of rapid generation of cool non-toxic inflation gas and long-term operability depend to a large extent on the gas generant selected and the physical form into which it is initially compounded.
If a suitable propellant can be designed, then the design of a complete passive restraint system undertaken with consideration of the characteristics of a particular propellant stands a better chance of practical success.
Naturally, from every point of view, the most desirable atmosphere inside an inflated crash bag would correspond in composition to the air outside it. This has thus far proven impractical of attainment. The next best solution is inflation with a physiologically inert or at least innocuous gas. The most practical of these gases has proven to be nitrogen. The most popular means of generating nitrogen has been the decomposition of alkali metal, alkaline earth metal and aluminum derivatives of hydrazoic acid, especially sodium azide.
Decomposition of these azides in the absence of other ingredients can only be accomplished by means of a high heat source and the decompositions are not self-sustaining. So as to provide favorable kinetic conditions for the decomposition, various co-reactants have been suggested. It is evident that such co-reactants must be selected to provide that the non-gaseous decomposition products are capable of containment or easily converted to manageable form. Non-gaseous decomposition products being those which may be solid or liquid at 25.degree. to 40.degree. C. Among the by products of the decomposition which must be contained or converted to a containable form is the free metal derived from the counter-ion in the azide salt. A number of co-reactants to provide cool sustained combustion together with formation of little or no free counter-ion metal have been proposed. The most notable of these are ferric oxide (U.S. Pat. Nos. 4,062,708; 3,931,040; 3,996,079 and 3,895,098) and a mixture of molybdenum sulfide and sulfur (U.S. Pat. No. 3,741,585), both of which systems, while usable in crash restraints, have proven to have inherent disadvantages to their commercial practice.
The molybdenum disulfide sulfur systems burn with the requisite speed, give cool gas when employed in a container of proper design and can be easily pelletized to stable pellets which are abrasion and vibration resistant for the expected life of the inflators. They do present an odor problem apparently because of the presence of trace sulfur compounds in the gas, the solid combustion residue is finely divided making containment thereof a difficult engineering problem, and the gas is generated at high pressures, over 2,000 pounds per square inch absolute (psia), requiring heavy walled vessels for its containment. Iron oxide-azide systems on the other hand are extremely difficult to compact to stable pellets in actual practice, they are cool burning, but also generate their nitrogen at pressures over 2,000 psia and tend to be unstable and slow burners. Their residue on the other hand is described in the prior art as being in the form of large particles of "clinkers" that are simpler to contain, and having no sulfur, they are naturally free of any sulfur odor problem.
Containment of the hot combustion residues within the gas generator is necessary to prevent them from damaging the fabric of the gas bag itself and to prevent them from coming in contact with and injuring the occupants of the vehicle.
Those gas generants of the prior art which provide a finely divided combustion residue have been indicated in that art as requiring elaborate and for the most part expensive filtering devices. None of these devices has proven sufficiently attractive for commercial practice. The force of the combustion gases, particularly in the initial stages of combustion, results in a substantial pass through of high temperature particulates to the exterior of the gas generator. The U.S. Pat. No. 4,062,708 discloses that the compositions contained therein leave fused coherent combustion residues which are relatively easy to contain with relatively simple filtration means. The U.S. Pat. No. 3,996,079 also indicates that combinations of azides and iron or nickel oxides will form a sintered residue on combustion, thereby simplifying containment of said residue. Unfortunately, for the reasons discussed herein, simple azide-oxide systems have not as yet demonstrated commercial value as gas generants.
Azide-metal oxide systems are somewhat slower burning than azide-molybdenum disulfide-sulfur systems although combinations within at least the upper limits of the acceptable combustion rate range can apparently be formulated. The incorporation of metal oxides into azide-molybdenum disulfide-sulfur systems would appear on its face as a means to obtain more rapid burning and an easily trapped sintered residue. Surprisingly, while such a combination is rapid burning, it is also substantially cooler burning, has no substantial objectionable odor in the gas generated and in the absence of a properly designed container package, does not form a sintered residue. In the absence of such a sintered residue, the elaborate filtration and neutralization devices of the prior art are required for use of this type propellant as a crash bag inflator. However, when used in the specially designed gas generator of the hereinafter identified Adams-Schneiter application, the azide-metal oxide-molybdenum disulfide-sulfur generant compositions can be caused to form a sintered residue enabling simplified neutralization and filtration of the gas produced by their combustion.
The importance of pelletization will be immediately apparent to anyone who understands that, all else being equal, the surface area of a gas generant determines its gas production rate. This surface area depends on pellet size and for reproducibility must be uniform. In the absence of pelleting, routine handling will cause abrasion of individual particles changing the burn characteristics of a generant batch unpredictably. Long term vibration stresses will cause a generant which is a mixture of ground components to separate according to the density of individual components, again making performance unpredictable. The use of organic binders for pelleting, while convenient, is not acceptable to automobile manufacturers because the presence of carbon containing compounds immediately introduces carbon monoxide into the combustion gas and frequently the presence of compounds containing carbon, nitrogen and hydrogen will lead to formation of HCN.
The use of an inorganic lubricant and binder for pelleting is therefore indicated. A proven lubricant and binder is molybdenum disulfide. Simple replacement of a portion of the iron oxide of iron oxide azide systems with sufficient molybdenum disulfide for satisfactory pellet manufacture of even greater quantities is not a direct solution because the resultant compositions are either slow burning, difficult to ignite, or just marginally acceptable (burn rates on soda straw size strands of 0.7 in. per sec. at 1000 psia pressure). A number of sources, including U.S. Pat. No. 4,062,708, suggest that the inclusion of perchlorate accelerators in azide-iron oxide systems will increase the burn rate. Unfortunately, the formation of hydrogen chloride and chlorine is a known problem of perchlorates.
The present invention provides a gas generant consisting of a mixture of non-explosive azides, iron oxide, molybdenum disulfide and sulfur which is easily pelletizable, surprisingly cool burning, generates nitrogen at much lower pressure than any prior art gas generant, has burn rates well within acceptable limits, sustains burning reliably after ignition, has very little or no odor in the nitrogen generated, and in a properly designed container leaves a combustion residue which is principally an easily contained "clinker" or fused mass.